This invention relates to marine seismic surveying by the use of an array of seismic air guns and more particularly to a towing armored umbilical for towing a sub-array of air guns from a survey vessel incorporating within the towing umbilical the high pressure air and the electrical control cables needed to operate the air guns making up the sub-array.
In seismic surveying in a medium such as water acoustic energy is generated by air guns sumberged in the water and is utilized to investigate subsurface geological conditions. For this purpose one or more of such air guns are submerged in the water and compressed air under pressure is fed to the submerged guns and temporarily stored therein. At the desired instant, the seismic source air guns are activated and fired thus abruptly releasing the pressurized air into the surrounding water. In this manner, powerful acoustic waves are generated capable of penetrating deeply into the subsurface materials of the earth to be reflected and refracted therein by the various strata and formations. These reflected or refracted acoustical waves are sensed and recorded to provide information and data about the geological conditions and formations.
Until recent times the air gun array was comprised of one or two sub-arrays towed behind the survey vessel forming the marine seismic source. It has become recently desirable, in poor marine seismic recording areas, to discriminate against unwanted reflection signals by forming a large array pattern of air guns in the water. The use of a large array in marine seismic surveying offers more directional control over the outgoing seismic energy and therefore can improve the recorded seismic data. This method of distributing the energy source over a large area to improve the directionality of the outgoing energy wave is known as pattern shooting and has been successfully used in land seismic surveying for the past thirty-five years. Usually the large array is formed by towing from the survey vessel a large number of individual sub-arrays consisting of a multiplicity of air guns. Paravanes are incorporated on the outside sub-arrays to help spread out the sub-arrays from behind the survey vessel, thus creating a large array of air guns.
With the need to tow large numbers of air gun sub-arrays in the water, it has become necessary to improve the towing means from the survey vessel to the sub-arrays. The present umbilicals used in the towing of sub-arrays are large and cumbersome and subject to damage from handling due to the lack of protection for the air supply lines and electrical control lines. The general method used today to tow air gun sub-arrays and to supply the air and electrical control lines to the air guns is to use a single steel wire rope as the tow line and a bundle of individual air hose lines and electrical cables bundled together to form the umbilical. This method results in an umbilical of large diameter with no protection to the air hoses and electrical cables from damage due to constant handling problems encountered in towing large numbers of sub-arrays in rough seas. A high percentage of air gun failure are due to air hose failures outside of the air gun.
A towing umbilical having a large outside diameter presents an additional problem when the type of array used necessitates that a number of sub-arrays be towed out on paravanes to extreme distances from the side of the survey vessel. When these sub-arrays are spread out behind the surveying vessel, the drag on each large diameter umbilical as it is towed sideways through the water, works in opposition to the action of the paravane and therefore limits the distance that the arrays can be offset from the towing vessel.
With the need for sub-arrays to be towed at increasing distances from the side of the survey vessel, it has become necessary that the towing umbilicals be of a small diameter to reduce the drag on the umbilicals but still retain the functional ability to tow the array and still supply the high pressure air and electrical control signals to the individual air guns within the sub-array.